Originally posted by Uluru
Registration distance in Pentax 645 system is around 71 mm. Plus, the MF lenses are heavier, they need a good counter balance.
It would be a shame to make 645 into a narrower and taller DSLR. Leica's S is like that, but I like handling of 645D much more.
Horses for courses.
There has been a lot of crazy talk about doing away with K-mount for a new mount with a shorter registration distance. As I've indicated, I think that this is not going to happen (though I've been wrong before).
Having said that, I do rather like the idea of Pentax adopting this strategy in the MF space. Develop a new, short mount suitable for mirrorless MF, with 645-compatibility via an adaptor. This would allow for the creation of something rather like Leica's S2, ideally at a price that would tempt FF shooters to upgrade.
As for those handling issues you've mentioned, I'm sure that they could be dealt with via some thoughtful design (such as a deep grip and a rearward weight bias in the body).